OS X: If you don't want to shell out hundreds of dollars for Photoshop, Pixelmator is a great alternative for the Mac. It's usually $30, but right now its on sale for $15.

No word on how long this holiday sale will last, so if you've been hemming and hawing about a good photo editor, now's a good time to jump on Pixelmator—$15 is pretty cheap for how good it is. It won't beat Photoshop in a battle of image editing prowess, but for $675 less, it's the best deal you're going to find.